Accommodation is the ability of the eye to create a sharp image on the retina of an object located at almost any given distance. Thereto, the required adjustment of the refractive power occurs essentially through the elastic deformation of the lens. The possible maximum change in refractive power is called amplitude of accommodation. It can amount to 16 diopters. Due to age-related hardening and/or thickening of the crystalline lens, its amplitude of accommodation decreases. This is called presbyopia or age-related farsightedness. Typically, a crystalline lens is called presbyopic when its amplitude of accommodation drops below 3 diopters. Presbyopia is not a pathological process but a natural sign of old age, starting approximately at age 40. Today, it is principally accepted that a change of the lens substance with continuous growth, i.e., an increase in thickness with age, causes an increase of the lens hardness, therefore constituting a limiting factor of the accommodation. Presumably, the cause for the altered lens elasticity is a sclerosis of the nucleus and the cortex of the crystalline lens.
In ophthalmology it has been suggested to restore improved deformability of a hardened lens through suitable incisions or creation of bubbles by means of a laser surgical therapy, particularly photodisruption (laser-induced optical breakdown or LIOB) or other incisions. Thereby, the accommodative capacity of the lens is to be partially regenerated. In EP 1 212 022 B1 and in US 2004/0199149 A1, such an approach is described using a femtosecond laser.
For example, WO 2005/070358 A1 describes methods for executing different cohesive cut surfaces for producing slip planes. An expanded ophthalmological laser system for presbyopia therapy, e.g., is disclosed in WO 2008/017428. It refers to a navigation apparatus for the optical analysis and processing of the inner structure of the crystalline lens. The navigation apparatus is provided with a confocal detection device. The same laser is provided for the illumination for analyzing of the inner structure as well as for processing. For analysis, the laser light, which is backscattered in the crystalline lens, is detected in order to determine position, geometry, and structure of the crystalline lens. The cutting geometries to be produced during processing are determined with the aid of the detected inner structures and the individual geometric shape of the crystalline lens. For said purpose, a basic model is adjusted to the detected individual geometry. US 2007/0173794 also describes a system for presbyopia therapy, wherein the cut structure is adjusted to the inner lens structure.
Recently, the tightly focused radiation of femtosecond lasers has also been used within the course of an intrastromal keratomileusis (LASIK) in order to produce incisions in the cornea (Femto-LASIK). Such devices are also called laser microkeratomes. Thereby, a photodisruption is produced in the focus, which leads to a minimal formation of bubbles in the stromal tissue. If focal spot is set next to focal spot by means of a scanner system, random incisions (perforations) can be made in the cornea. Such incisions are hereinafter called laser incisions. For example, it is known from US 2006/0155265 A1 to cut the flap by means of a femtosecond laser system. The ablation of the stromal tissue, necessary for a refractive correction, is subsequently executed conservatively by means of an excimer laser, and therefore a mechanical treatment can be completely foregone. In WO 2008/064771 A1, a femtosecond laser system is described which can also prepare the flap but is additionally capable of separating the ablation of stromal tissue, necessary for a refractive correction, through dual incisions for the preparation of a lenticle. This can be called femtosecond lenticle extraction. Subsequently, the lenticle can be removed with a pair of foreceps after opening the flap. Therefore, only one laser system is required, the use of an excimer laser can be forgone.
It is problematic that the visual perception can be impaired with all the above-mentioned methods due to a spectral splitting of incidental white light (Krueger, Thornton, Xu, Bor, and van den Berg: “Rainbow Glare as an Optical Side Effect of IntraLasik,” Ophthalmology, 2008, 115 (7)). The cause seems to be defect areas which form after the photodisruption through the femtosecond laser at the focal positions and act as transmission dot arrays. The grid structure on the corneal flaps occasionally remains in place. The intensity of the effect can be reduced through the use of focusing optics with greater numerical aperture because smaller disruption bubbles are produced as a result.